In this grant's competitive renewal (years 25-29), we will test the hypothesis that Hypoxia Inducible Factor (HIF) regulates endothelial barrier repair a the level of adherens junctions (AJs) through transcriptional expression of the AJ proteins Vascular Endothelial (VE)-cadherin and Vascular Endothelial-Protein Tyrosine Phosphatase (VE-PTP). We will take advantage of mouse models in which expression of genes in endothelial cells are modified and which we have on-hand [e.g., endothelial cell-specific and inducible disruption of Hif1? or Hif2? genes]. We will investigate mechanisms of VE-cadherin and VE-PTP expression and their important role in homeostatically restoring lung AJ integrity and tissue fluid balance and polymorphonuclear leukocyte (PMN) trafficking in lungs. The specific aims of the proposed studies are: AIM 1: To determine the role of HIF-dependent expression of VE-cadherin and VE-PTP as an adaptive mechanism that induces AJ strengthening and re-sealing and thereby restores endothelial barrier integrity and lung fluid balance. Hypothesis: HIF-induced VE-cadherin and VE-PTP expression after inflammatory lung injury leads to strengthening of the AJ barrier and increased restrictiveness of lung endothelial barrier to plasma proteins and inflammatory cells. This HIF activated feedback mechanism functions to restore lung endothelial barrier function and fluid balance. Here we will determine whether HIF1? and HIF2? have the complementary function on AJs in enhancing barrier recovery after injury. We will also address the concept that full barrier restoration requires re-sealing of the A barrier secondary to activation of the RhoGTPases Rac1 and Cdc42 in addition to the AJ strengthening function of HIF transcriptional activity. AIM 2: To determine the role of prolyl hydroxylase-2 (PHD2), a druggable target, using genetically modified mice in strengthening endothelial AJs through stabilization of HIFs, and thereby in restoring lung vascular integrity and normalizing trans-endothelial trafficking of inflammatory cells. Hypothesis: PHD2 inhibition functions by stabilizing HIFs and thereby restores endothelial AJ integrity secondary to expression of VE-cadherin and VE-PTP and prevents vascular injury and inflammation. With the completion of these aims, we hope to define the mechanisms by which HIF signaling promotes lung vascular barrier integrity and fluid balance and whether PHDs serve as targets to prevent persistent lung vascular leakiness and edema associated with inflammatory lung injury.